The detection and identification of materials (e.g., chemicals) at a distance is important in various applications and industries such as the health industry, the security and defense industries, or the manufacturing industry. Techniques may be used to identify dangerous or hazardous substances, detect substances that may be helpful or beneficial, or identify contaminants in a sample.
Methods for the detection of trace chemicals on surfaces at a distance (e.g., standoff detection) may fall into two categories: high power point detection and thermal emission detection. High power point detection category may include both frequency resolved Raman scattering and laser induced breakdown spectroscopy (LIBS). Both of these methods may be implemented using lasers that are capable of high peak powers. These lasers, however may be harmful to humans. Furthermore, because of the use lasers with of high peak powers and the relatively weak signals they produce for detection in the Raman scattering and LIBS methods, the sensitivity of both methods may be very dependent on the distance between the object that the lasers are directed at and a sensor detecting the signals produced by the lasers (e.g., the standoff distance). The lasers capable of high peak powers used in both of these detection techniques also may require a focused laser beam and may limit them to point detection and powers that can be unsafe for humans (and also can be destructive in the case of laser used in the LIBS method). Accordingly, in view of the use of point detection with these two methods, imaging large areas may not be efficient or practical.
Thermal emission detection methods may rely on a laser heating a chemical and detecting a temperature change via thermal emission signals. However, the practicality of deploying systems that use these methods may be hindered because the thermal emission may be weak, as only a fraction of the energy absorbed by a sample may be converted to thermal emission, especially if a contaminant is spread out over the environment. Thermal emissions are also isotropic. Accordingly, the detection of thermal emission may only be sensitive to range according to a typical 1/R2 manner, where R is the range. Furthermore, detection of thermal emission at wavelengths between 8-14 micrometers (μm) may require detectors that are slower and more expensive relative to technology in the visible spectrum and near infrared (NIR) spectrum (e.g., just under 800 nanometers to just over 1 micrometer). Many of the thermal emission detectors may also need to be actively cooled, which can increase the cost and power consumption of such devices. Thus, thermal emission detection may require larger receiver apertures to capture small signals even at short ranges, and the collected emission may need to be detected by expensive, slow, actively cooled long wave infrared (LWIR) detectors.